TB3 Lecture 2; Vision

Vision in sensory processing

• 1) The neural retina
  • In the retina, there are 5 different cell types. From the most external to internal, these are the 1. ganglion cells, 2. amacrine cells, 3. bipolar cells, 4. horizontal cells and 5. photoreceptors. Light detection occurs at the final layer; rods and cones.
  • There are 3  types of cone, which are sensitive to three types of wavelength, short, medium and long. There is only one type/sensitivity of rod! ):

• 2) How are photons (light) converted into neural signal?

1.Photons hit Rhodopsin; Inside the photoreceptor, there are discs, perforated with rhodopsin molecules. Bound to the rhodopsin molecules is retinal (a derivative of vitamin A) which changes shape (conformation) after absorbing a photon. In this way, the photoreceptor 'gathers' the light and turns it into a signal.

• When the rhodopsin molecule is inactive, it keeps cGMP in balance. cGMP is a second messenger and it regulates the conductance of sodium ion channels in the photoreceptor membrane.
  • Because of this, when light activates rhodopsin, cGMP is broken down, causing the sodium ion channels to close. 
    • This changes the action potential of the cell, and reduces the release of the neurotransmitter glutamate.

2. A signal cascade chain occurs through the other cell types; Because of the change in glutamate release, the action potentials of the other cells in the retina also changes. This looks like a signal cascade through all the other cell types, this communication within the retina is through graded potentials; cells hyperpolarize according to different light intensities.

3. An action potential is released from the retina as the final output; Bipolar cells may sample over a number of different cones, allowing an 'average' sensitivity to be generated from this selection. There is then a single ganglion cell over a few bipolar cells in the same process, these bipolar cells directly synapse with the retinal ganglion cells,which…